Today I'm using a Sony Alpha380. It was my first DSLR and now after 2 years I feel that I want to make a run for it with my photography. I want to upgrade, BUT sould I keep on using Sony? I love my DSLR and I have absolutely no complaints, but using a Canon would make it easier to find equipment, especially since I'm just a poor student!

Today I have the standard kit lens 18-55 and a 70-300 sigma lens. If I just bought an Sony Alpha 580 body, I would still have my lenses to use.
If I buy a Canon I can't afford anything more expensive than the 550D with a kit lens (18-55).

I can't think of any reason to switch unless you were unhappy with the experience, the ergonomics, or the controls. You don't have a huge or expensive commitment in lenses yet, so that's not too much of an issue - but if you're severely budget limited that can make a difference.

Canons are fine cameras, as are any other brands. The 550D would be a small step up - not much more. And with only the kit lens, you'd be much more limited in your lens range until you spent more money. Canon has moved on to the T3/600D which is a better camera, but obviously more money still.

The Sony A580 on the other hand would be a very large step up from your current camera, and is a fairly significant step on the 550D as well. Being able to move to the A580 and still have two lenses to start with right off the bat puts you in much better overall shape for versatility and ability to expand your photography into new areas. It's MUCH faster - both in focus and continuous frame rate, the focus system is better, tracks better, has more focus points, the live view system is a huge upgrade and the best live view system ever developed by a mile, and the high ISO performance is tops for APS-C, with the Sony-made 16MP sensor in the A580, Nikon D7000, and Pentax K5 consistently top ranked - and those two cameras are much more expensive. With more and better controls than your current camera, huge battery life, and gorgeous high-res LCD, it's clearly in another class from your current cam.

By the way, as far as lenses and equipment - it shouldn't be as hard to find equipment as you make out...have you shopped around for Minolta lenses to go with your Alpha? Some amazing, wonderful lenses out there in Minolta land, as good or better than a lot of the Sony stuff, and very cheap on the used market. These screw-driven lenses tend to be reliable and less prone to worry about dying motors or bad stabilization systems in the lenses, since none of that exists - it's all in the camera body. Many Sony users are not aware of the fact that Minolta autofocus lenses work exactly like new on Sony's DSLRs, and think the lens selection is a little smallish (105 current lenses for Alpha mount)...adding Minolta to the list opens you up to over 360 lenses...and some very cheap. I'd consider adding one fast prime to your collection - you can pick up a Minolta 50mm F1.7 for $70 or under...if the newer Sony options are too much.

The main reason for me to change body is because I have developed rheumatism in my hands. My a380 has a quite unique grip, and it doesn't work out for me anymore. My hand is hurting just after a few minutes. I haven't had that problem with Canon, Nikon or other Sony cameras with a more "traditional" grip.

That's reasonable. It might be worth the switch, and fortunately the Sony grip experiment of those models has ended, and they have stuck with good, deep, traditional grips. In fact, one reason I ended up with Sony was that I found their grips the most comfortable, deepest, and easiest to carry hanging from my fingertips without having to clamp down on the camera...but I've always had the more traditional-gripped Sonys (A300, A550). I would go test out the Canons and make sure they're OK with you - of Sony, Nikon, and Canon, I find the entry-level Canons to be the least comfortable for exactly the reason your camera was uncomfortable for me - the grip is way too shallow, with not enough depth for finger-wrapping. I'm OK with Nikon grips and Sony grips, but struggle with entry Canons and Olympus bodies. Pro-body Canons are much better, with a much more prominent grip design. Pentaxs are a little smallish for my hands too, but the grip is a little better pronounced than Canon. I'd think if you struggle with the A380, that the entry-level Canons like the 550D would not really alleviate the problem in the long run.

The main reason to go with Canon is probably for their video mode. For everything else, I think all the camera manufacturers are about equivalent. So unless you want to also do some videography, there's nothing wrong with sticking with Sony, especially if you can't afford the t2i with kit lens & telephoto lens.